Add water and mix until combined. Knead for 6 minutes by hand or 4 minutes with an electric mixer. The dough should be loose and wet.
Let rise for 4-5 hours at room temperature, punching down halfway. Or let rise overnight (for up to 3 days) in the refrigerator.
For the Potato Bread
Combine flour, salt, yeast, and spices.
Add the olive oil, mashed potatoes, and the preferment. Mix just slightly.
Add the water, mixing until the dough is homogenous.
Knead 8 minutes by hand or 6 minutes with the electric mixer.
Cover and let rise until doubled, about an hour.
Once the dough has risen, divide it into two portions.
Shape each portion into a sphere. To get the dough to stay this shape while rising and baking, pull the edges down and push them up into the center like you're tucking in the excess underneath and up under the rest. Repeat this with both portions until the dough is taught and holds a sphere shape easily.
Let rise until doubled, about an hour. I like to let the dough rise on parchment at this point so I can easily transfer it into the oven. If you don't have a baking stone, place the dough on lined baking sheets to rise
Preheat oven to 400ºF. Fill a 9x13 baking dish halfway with water and place it on the top rack of the oven. This will help create the crust on the bread while it bakes.
Gently brush each loaf with olive oil and sprinkle with fresh rosemary and coarse salt, if desired.
Bake for 30-35 minutes on a baking stone or until the center of the loaf registers 195º-200º on an instant-read thermometer (insert it from the side so you don't ruin the aesthetic of your loaf).
Remove from the oven and let cool at least 1 hour before slicing.
If you don't have a baking stone, use an overturned cookie sheet to preheat in the oven for baking. Bread flour needs to be used in this recipe, substituting other flour will result in a flatter loaf with less artisan texture. The baking dish with water helps create the crispy crust on the outside of the bread. Fresh herbs can be substituted, use 1 teaspoon of each. If you do not have instant yeast, you can use active dry. The rise steps just may take a little bit longer. It is not necessary to bloom the yeast in water.